Among the systems for cooling the electric motors of electric or hybrid vehicles, it is known to use the lubrication oil of the transmission system constituted by the speed reducer that is conventionally coupled to the electric motor. The use of the lubrication oil of the speed reducer for direct cooling of the heating elements, such as coils, magnets, sheets, of the electric motor in fact constitutes a relatively effective solution compared to the other known solutions such as those which utilize cooling with water circulating around the housing of the electric motor, or even cooling with air, to the extent that the oil may be in direct contact with all the components to be cooled. The use of the lubrication oil of the speed reducer to cool the heating elements of the electric motor further permits the friction of the transmission system when cold to be reduced because the lubrication oil introduced into the transmission system in this way is less viscous as a result of being heated in the electric motor. Particularly well known is document CN203645464, which describes a motor and a speed reducer that are cooled by the same oil circulating in a circuit combining the cooling oil of the motor and the lubrication oil of the speed reducer.
However, these cooling systems require the removal of the heat stored by the oil that is used jointly for cooling the motor and for lubrication of the speed reducer. A heat exchanger of the oil/air radiator type disposed on the front face of the vehicle is used for this purpose, or alternatively an oil/water exchanger mounted at the level of the power train and supplemented by a water circuit and a water/air exchange radiator on the front face of the vehicle. As a variant, the calories generated in the motor and in the transmission system may be removed by a flow of air, notably a flow of air exiting from the fan on the front face of the air conditioning condenser for the passenger compartment of the vehicle. This flow of air, which may be ventilated either by this fan or by the forward movement of the vehicle, circulates around the assembly constituted by the motor housing and the transmission system in order to ensure the desired heat exchange.
It has been established that, in the case of high-performance vehicles, there is a need to increase the heat exchange performance between the air and the housing of the electric power train of the vehicle.
Furthermore, when the assembly of the electric motor and its associated speed reducer produce a large amount of calories, it is desirable for the temperature of the oil to be capable of being higher in order to facilitate the removal of the thermal energy produced in the direction of the air heat exchanger. However, if the speed reducer may be lubricated by an oil at a temperature in the order of 130-145° C., in the context of a cooling solution having an oil circulation circuit combining the engine oil and the lubrication oil of the speed reducer, the temperature of the latter is limited by the operating constraints of the motor which impose an oil temperature in the order of 90-100° C. In addition, when hot, that is to say when the electric power train is producing a large amount of thermal energy, the operating temperature of the speed reducer is limited, which has an adverse impact on the cooling capacity of the ventilated air passing around the housing of the power train. In fact, the greater the difference in temperature between this ventilated air and the housing of the power train exposed to this flow of air, the better the global cooling performance of the speed reducer and of the assembly of the electric motor and its associated speed reducer.
The need also exists for a device for the thermal management of an electric power train, which, while retaining the advantages associated with the combination of the engine oil and the oil of the speed reducer, in particular during operation of the power train when cold, overcomes the aforementioned disadvantages, in particular during operation of the power train when hot.